This invention relates to the removal of carbon dioxide in geothermal power generating processes employing a geothermal fluid such as geothermal brine containing CO.sub.2, to improve the efficiency of geothermal power generation, and is particularly concerned with a process and system of the above type, e.g., employing direct contact heat exchange between a hot geothermal fluid and a working fluid such as a hydrocarbon, wherein vapors containing working fluid and uncondensible gas including carbon dioxide, are treated with a chemical treating agent to remove the carbon dioxide (and other undesirable constituents, if present, such as SO.sub.2 or H.sub.2 S), and reduce working fluid losses, and improving the efficiency of geothermal power generation.
U.S. Pat. No. 3,988,895 to Sheinbaum, and application Ser. No. 589,068, filed June 23, 1975, by Samuel G. Woinsky now abandoned in favor of U.S. Application Ser. No. 76,677, filed Sept. 19, 1979, now U.S. Pat. No. 4,324,102 (the disclosure of which is incorporated herein), disclose recovery of energy from geothermal brine by passing such geothermal brine under pressure into direct heat exchange contact with a working fluid such as isobutane, expanding the resulting hot working fluid or isobutane in a work expander or a turbine to produce work, condensing the turbine discharge, and returning the resulting condensed working fluid to the heat exchanger, and withdrawing the spent brine from the column and discarding same as by introducing the spent brine into a brine injection well.
The geothermal brine generally contains uncondensible gas, including carbon dioxide, as well as other uncondensible gasses such as a small amount of hydrogen sulfide. Non-condensible gasses such as carbon dioxide are responsible for appreciable losses in the efficiency of geothermal power generating systems. It is thus desirable for increased efficiency of power generation and economical operation for geothermal power generation, to reduce the amount of uncondensible gas, particularly carbon dioxide, as much as possible (at a reasonable cost), in order to reduce working fluid losses resulting from venting of working fluid vapors together with the uncondensible gas, particularly CO.sub.2, during operation of the system. In the above patent, and in above U.S. Application Ser. No. 76,677, now U.S. Pat. No. 4,324,102, and also in U.S. Application Ser. No. 8,793, filed Feb. 1, 1979 by Samuel G. Woinsky, now U.S. Pat. No. 4,279,125 (the disclosure of which is incorporated herein), the geothermal fluid or brine is initially degassed, as by steam stripping or by flashing, whereby steam and uncondensible gas, including CO.sub.2, hydrogen sulfide and ammonia, which may be present in the geothermal fluid, is removed or flashed off. However, residual uncondensible gas including carbon dioxide, even under these conditions, remain in the geothermal fluid.
Thus, where the geothermal fluid introduced into the direct contact heat exchanger still contains uncondensible gas including CO.sub.2, such uncondensible gas including the CO.sub.2 is vented from the system following power generation, carrying along working fluid vapor, and thereby resulting in working fluid losses. Also, the presence of uncondensible gas including CO.sub.2 usually results in reduced efficiency of power generation. Also, spent geothermal fluid or brine from the direct contact heat exchanger generally contains working fluid and uncondensibles including CO.sub.2, and it is desirable to recover such working fluid, while removing any uncondensibles such as CO.sub.2.
U.S. Pat. No. 4,084,379 discloses an energy source fluid such as geothermal fluid for the production of power in which direct heat transfer is utilized by the hot fluid source to vaporize a working fluid which is then used for the production of power. In such system the energy source fluid contains a small portion of the working fluid which is lost with the spent energy producing fluid. The dissolved working fluid is removed from the spent energy source fluid by adding a salt such as calcium chloride to saturate the water so as to lower the soluability of the working fluid in the energy producing fluid by a "salting out" effect, and thereby remove the dissolved working fluid and reduce working fluid losses. Uncondensible gases dissolved in the energy source fluid are pumped out of the system following the power generation step.
It is accordingly an object of the present invention to provide a process and system for recovery of energy from geothermal brine and hot water sources, having improved efficiency of operation. Another object is the provision of a process and system as described above, wherein uncondensible gas, particularly carbon dioxide, contained in the geothermal fluid, is removed from the process streams obtained from the geothermal fluid during power generation. A particular object of the present invention is the provision of a procedure and system employing direct contact heat exchange between a geothermal fluid or geothermal brine, and a working fluid such as a hydrocarbon, wherein the amount of uncondensible gas, particularly carbon dioxide, present in the geothermal fluid is substantially reduced during operation of the process for generation of power, by removal of the carbon dioxide for the process streams employing a chemical agent, to thereby reduce working fluid losses discharge from the system with carbon dioxide and other uncondensibles, and enhance power generating efficiency.